Subject: [magma] IP multicast and Ethernet multicast

From: "Beatriz Silva" <[email-masked]>
Date: Thu, 27 Jun 2002 05:36:00 -0500

Hi everybody !

I am new in this list and I do not know if my question is too basic for the purpose of the list. If so, could you recomend me another list ?

Anyway, my question is the following:

Normal hosts have to be extended in order to deal with multicast addresses.
I read in RFC 1112 that IP multicast addresses are mapped to specific ethernet multicast addresses  (by placing the low-order 23 bits ...). 
How do switches then deal with the ethernet multicast addresses ? Do they somehow have to be extended ? 
And also, I do not understand why there is this map to an ethernet address. I thought that IP multicast addresses would be mapped to the mac addresses of the members of the group ?

As you can see, I am confused. Can somebody explain me what happens ?

Thanks,

Beatriz

-- 
__________________________________________________________
Sign-up for your own FREE Personalized E-mail at Mail.com
http://www.mail.com/?sr=signup

Save up to $160 by signing up for NetZero Platinum Internet service.
http://www.netzero.net/?refcd=N2P0602NEP8

_______________________________________________
magma mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/magma

---

From: Frank Solensky <[email-masked]>
Date: 27 Jun 2002 11:08:24 -0400

> Do they somehow have to be extended ?

In the simplest case, no: it just floods the packet the same way it
handles broadcast packets.  It can, however, add a feature that keeps
track of which ports are connected to nodes that are members of that
group and flood the packet only to those ports.  There's a draft in the
repository that goes into the details (draft-ietf-magma-snoop-01.txt);
I'm playing "beat the clock" to get the -02 version out by the cutoff
date (subliminal query to the group for any additional comments).

> And also, I do not understand why there is this map to an ethernet
> address. I thought that IP multicast addresses would be mapped to the
> mac addresses of the members of the group ?

In order to do it this way, the sender would need to keep track of which
nodes are members of the group and then send individual copies out. 
That defeats the purpose of multicasting: send one out that gets picked
up by only the interested parties.  The way the hardware makes this
distinction is by looking at only the first bit of the MAC address: a
'0' indicates only one receiver and '1' indicates a group address (the
special case of all 1s being a broadcast).
					-- Frank

---

From: Brian Haberman <[email-masked]>
Date: Thu, 27 Jun 2002 11:18:33 -0400

> special case of all 1s being a broadcast).

To expand a little on Frank's answer.  When a node decides to join
an IP multicast group, it must also register with its NIC that it
wants to listen for the corresponding L2 group address.

Brian